1. Field of the Invention
The present invention relates to a noise eliminating apparatus for a video signal and, more particularly, to a noise eliminating apparatus which comprises a recursive digital filter using a frame memory and eliminates noise components contained in the video signal.
2. Description of the Related Background Art
The video signal is a signal in which image information is repeated at a frame period and the correlation between the frames is very strong. On the other hand, the noise components included in the video signal generally do not have the frame correlation. When, therefore, the video signal is averaged every frame period with respect to the time, the energy of the signal component hardly changes and only the energy of the noise components decreases. There is known a noise eliminating apparatus which has been made in consideration of such a fact. An example of a fundamental arrangement of the noise eliminating apparatus is shown in FIG. 1.
In FIG. 1, a video signal which is extracted from the broadcasting waves or obtained by reading out of a recording medium is digitized by an A/D (analog/digital) converter 1 and is supplied to a subtractor 2. A subtracted output of the subtractor 2 is multiplied with a recursive coefficient .alpha. by a multiplier 3 and is supplied to an adder 4. An added output from the adder 4 is directly converted into an analog signal by a D/A (digital/analog) converter 5 which is a video signal to be supplied to a succeeding stage. The added output from the adder 4 is also supplied to a frame memory 6 and the video data of one frame is stored therein, so that this output is delayed by only the period of time corresponding to one frame. The delayed video data are sequentially supplied to a chroma inverter 7, which inverts the chroma component on the basis of the phase inversion of the chroma component of each frame in the NTSC system so as to coincide the chroma phases of the neighboring frames. Thereafter, the resultant output signal is supplied to the other input terminal of each of the subtractor 2 and adder 4. In this manner, a recursive digital filter 8A using the frame memory is formed.
In the recursive digital filter 8A, the recursive coefficient .alpha. has a value within a range of 0&lt;.alpha..ltoreq.1. When .alpha.=1, the noise eliminating effect is not obtained and an input video signal .alpha. directly becomes an output video signal v. On the contrary, when .alpha..apprxeq.0, a full feedback recursive digital filter is formed and the largest noise eliminating effect is derived. FIG. 2 shows an input/output gain characteristic graph of the recursive digital filter 8A when .alpha.=0.5. As being apparent from this graph, the recursive digital filter 8A has a band eliminator characteristic in the time direction (frame-to-frame direction) and can eliminate the noises in the hatched region in FIG. 2.
On the other hand, it is natural that an image represented by a video signal moves and the correlation between the neighboring frames of the video signal in this moving portion is small. Therefore, when .alpha. is set to a small value, if the motion components exist in the hatched region in FIG. 2 in the moving portion, the motion components are attenuated, so that what is called a dimming phenomenon occurs on the display screen due to the motion. To avoid such dimming phenomenon, there is provided a motion detector 9 to detect the motion of the image on the basis of a difference signal p between the neighboring frames as a subtracted output of the subtractor 2, thereby controlling the value of .alpha. in accordance with the level of the difference signal p. In the still image portion, by setting .alpha. to a small value, the noises can be eliminated. In the moving portion, by setting .alpha. to a large value, the dimming phenomenon can be suppressed.
However, in the conventional noise eliminating apparatus with the foregoing arrangement, the noise eliminating efficiency must be sacrificed in order to suppress the deterioration in picture quality resulted from the dimming phenomenon in the moving portion. Therefore, it is difficult to simultaneously perform sufficient noise eliminating functions while avoiding the dimming due to the motion. In other words, when .alpha. is set to a large value in the moving portion, the noise eliminating efficiency is sacrificed, causing a phenomenon such that the noise images or spots become conspicuous in the video signal.